Low-level detection of illicit substances including narcotics and organophosphorus compounds (OPCs) remains a long-standing goal of modern instrument platforms. For example, detection of drugs at ultra-low levels significantly below those currently detected by state-of-the-art instrument systems analyzing swipe samples could enable vapor detection that would compete with a detection acuity on the order of sniffing dogs. Such systems would enable detection of vapors to identify illicit drugs in cargo containers and other concealed conveyances. And, ultra-low level detection of OPCs could also allow identification of hazardous compounds at levels well below those considered safe for the general population. However, positive detection of OPCs, for example, at these ultra-low concentrations can be masked by presence of other trace-level analytes in samples being tested. Accordingly, new methods are needed that provide accurate detection of OPCs and other important analytes at these ultra-low levels. The present invention addresses this need.